The invention relates to a granular body and a method of manufacturing it, especially of SiO.sub.2 for an article having a nonuniform refractive index.
Enlongated glass bodies can be used as so-called preforms for the production of optic fibers having nonuniform transverse refractive indices, preferably in the visible spectrum or in the short infrared wavelength range, for communication. In addition, such bodies can also be used for the production of glass rods or glass plates which have lens-like properties.
For optic fibers used for communication, the refractive index in the cross section of the fiber is greater at the core of the fiber than in the peripheral part thereof. In the case of the so-called monomode fibers and the so-called stepped index fibers, the refractive index transition between the axial or core area and the peripheral area or "cladding" is abrupt. Optical waves which propagate away from the core of the fiber are reflected back into the core at this abrupt transition. In the so-called multimode fibers, the refractive index gradually decreases from the core to the peripheral area, preferably in an exponential manner. In this case, waves which do not propagate parallel to the axis of the fiber are gradually turned toward the axis by a kind of lens effect to remain captive in the core or axial region.
For the rods and plates with lens-like properties mentioned above, a gradual reduction of the refractive index from the center of the body to the margin is prescribed, and again it should be exponential. Light rays passing through a parallel-sided plate of such glass are increasingly deflected toward the center such that the plate behaves optically like a lens.
In both applications, but especially in communications fibers because of the length of glass through which the rays must propagate, the precision of the glass must be very high with the absorption and scatter of the optical rays reduced to a minimum. Consequently, in the production of the fibers, the utmost purity must be achieved since residual absorption is due mostly to the presence of foreign substances. Moreover, the profile of the refractive index must follow the prescribed form very closely. The desired properties can be achieved, for example, by the use of high-purity fused silica as the basic material with other materials added to it, such as titanium dioxide, germanium oxide, fluorine, boron trioxide, and phosphorus pentoxide, to vary the refractive index of refraction. It has been found advantageous to use more than one dopant to achieve the desired properties. In this manner, for example, the internal tension due to a dopant can be adjusted to a desired level. It has also been found that the useful signal bandwidth of the fibers can be influenced to a great extent by precise control of the refractive index profile. For this purpose it can be advantageous to use refractive index profiles other than the exponential. For example, a step at the margin of an exponential index profile of the core has proven practical.
In the known processes, as disclosed, for example, in German Patent 2,715,333 or Offenlegungsschrift 2,313,203, it is possible in a simple manner only to produce cylindrically symmetric refractive index profiles, resulting in fibers of circular core cross section and circular fiber cross section. Although such fibers are suitable for communication, they have certain disadvantages. For example, they have no preference for a particular polarization of the light which they transmit. This is due to the wave-guiding nature of the optical fibers. A preference for certain polarizations is then produced by fortuitous tension distributions in the fiber, but these fluctuate rapidly and to a very great degree in practical operation, due to external influences, such as acoustic or mechanical influences.
It is also desirable to produce fibers with multiple cores, so that a plurality of communication channels can be contained in a single fiber. It has been possible to produce such fibers heretofore only from materials, such as low-melting glasses, and by methods, such as the multiple-crucible process, which do not result in the low attenuations necessary for communications.
The production of optic fibers is normally accomplished by first producing an elongated glass body having a refractive index profile that is similar to that which the later optic fibers are to have. This elongated glass body is then drawn at high temperature in to a fiber, while, due to the high viscosity of the material during the drawing process, the radial profile of the refractive index is largely preserved.
It is generally known that vitreous silica is used as the basic material for high quality communication fibers. It is also known that, theoretically, still greater bandwidths and still less attenuation can be achieved with other materials. These include, for example, a mixture of germanium oxide and antimony oxide or a whole series of known fluoride glasses. Also, optic fibers of more or less high attenuation can be produced from various crystals, plastics, and still other types of glass. The method of the invention is to be suitable for the use of all these materials equally.